i think the order of steepness should be 1 > 2 > 3.
i'm just using logic here:
accel is increasing from 0 to t1 hence vel is increasing at the fastest rate in this interval. from t1 to t2, accel is constant. so vel is increasing at a constant rate ( this rate < the rate from 0 to t1).
and from t2 to the end, accel is decreasing. so the vel is anyway decreasing. so less distance will be covered as compared to the former 2 intervals.

Displacement-time is:
|dw:1342520737774:dw|
The curve is always increasing as well as increasing in slope. Only the curvature becomes 0 at the end (would go on in a straight line if acceleration remains 0 or would reach an inflexion point if acceleration becomes negative after tfinal

The steepness (of displacement curve) is ever increasing because velocity is; so 3>2>1
But, as velocity is a continuous function, the 3 parts of the curve will smoothly
link together, without any angle at t1 and t2